The present invention relates to a pneumatic radial tire and more particularly to a pneumatic radial tire having improved durability adapted for use in heavy-duty vehicles such as buses or trucks.
The carcass line of a conventional heavy-duty pneumatic radial tire is formed so that the profile in such a state that the tire is mounted on a rim and filled with air corresponding to 10% of a normal internal pressure is approximated by a combination of arcs having a plurality of radii of curvature as shown in FIGS. 4 and 5.
Specifically, the profile of the carcass layer 4 of each of FIGS. 4 and 5 is formed by a combination of a radius of curvature R.sub.a of the crown portion, a radius of curvature R.sub.c of the shoulder portion, and a radius of curvature R.sub.L of the side wall portion 2 and characterized in that the radius of curvature R.sub.c of the shoulder portion is different from the radius of curvature R.sub.L of the side wall portion. Further, the radial tire shown in FIG. 4 satisfies the relationship of R.sub.c &lt; R.sub.L, while the radial tire shown in FIG. 5 satisfies the relationship of R.sub.c &gt; R.sub.L.
Studies conducted by the present inventors have revealed that the radial tire having the above-described carcass line profile brings about separation in an early stage because either of the edge of the belt layer 6 and the edge of the turnup portion of the carcass layer 4 in the bead portion 3 is weaker than the other and therefore has a short service life on the whole.
Specifically, in such a state that a tire to be filled with air rotates on a ground, when the radius of curvature R.sub.c of the shoulder portion is different from the radius of curvature R.sub.L of the side wall portion, the portion having a larger radius of curvature is more susceptible to the concentration of stress than the portion having a smaller radius of curvature, so that separation occurs. For example, in the case of a tire shown in FIG. 4 satisfying the relationship on the radius of curvature of R.sub.c &lt; R.sub.L, the separation occurs in an early stage at the portion having an edge of the turnup portion of the carcass layer 4 in the bead portion 3, while in the case of a tire shown in FIG. 5 satisfying the relationship on the radius of curvature of R.sub.c &gt; R.sub.L, the separation occurs in an early stage at the portion having an edge of the belt layer 6. This is because a portion with a large radius of curvature is urged to become large in a change of curvature when a tire deforms by rotation on a ground.